The small ad for the manga in the page of another comic.
It's been a while since this was even mentioned (was it even mentioned on this wiki? Hmm...) but last year in November it was announced in Square Enix's Monthly Shounen GanGan manga magazine that The 3rd Birthday would have a manga release just before the game was out. (In the pic there's The 3rd Birthday name with the words below it'll be released pre-game.) However, even now, I haven't heard or seen a thing about it.
Anyone know what happened to it? Or if anyone has the scans?